The present invention relates generally to radio communication systems and, more particularly, to techniques and structures for measuring interference associated with a signal received in a code division multiple access (CDMA) system.
Traditionally, radio communication systems have employed either Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) to allocate access to available radio spectrum. Both methods attempt to ensure that no two potentially interfering signals occupy the same frequency at the same time. For example, FDMA assigns different signals to different frequencies. TDMA assigns different signals to different time slots on the same frequency. TDMA methods reduce adjacent channel interference through the use of synchronization circuitry which gates the reception of information to prescribed time intervals.
In contrast, CDMA systems allow interfering signals to share the same frequency at the same time. More specifically, CDMA systems “spread” signals across a common communication channel by multiplying each signal with a unique spreading code sequence. The signals are then scrambled and transmitted on the common channel in overlapping fashion as a composite signal. Each mobile receiver correlates the composite signal with a respective unique despreading code sequence to thereby extract the signal addressed to it.
The signals which are not addressed to a mobile receiver in CDMA assume the role of interference. To achieve reliable reception of a signal, the ratio of the signal to the interference should be above a prescribed threshold for each mobile station (referred to as a “required signal-to-interference” level, or SIRreq). For example, as shown in FIG. 1A, consider the case where three mobile stations receive, respectively, three signals from a common CDMA communication band. Each of the signals has a corresponding energy associated therewith—namely energy levels E1, E2 and E3, respectively. The communication band also contains a certain level of noise (N). For the first mobile station to receive its intended signal, the ratio between E1 and the aggregate levels of E2, E3 and N should be above the first mobile's required signal-to-interference ratio.
To improve the signal-to-interference ratio for a mobile, the energy of the signal is increased to appropriate levels. However, increasing the energy associated with one mobile station increases the interference associated with other nearby mobile stations. As such, it is desirable for the radio communication system to strike a balance between the requirements of all mobile stations sharing the same common channel. A steady state condition is reached when the SIR requirements for all mobile stations within a given radio communication system are satisfied. Generally speaking, the balanced steady state may be achieved by transmitting to each mobile station using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high levels raises interference experienced at each mobile receiver, and limits the number of signals which may be successfully communicated on the common channel (i.e., it reduces system capacity).
Once a steady state condition is achieved, adjustments are made for various changes within the mobile communication system. For instance, when a new mobile station enters a communication cell, it creates additional interference within the system. For example, as illustrated in FIG. 1B, the introduction of a fourth mobile station to the steady state condition depicted in FIG. 1A imposes a new signal on the common communication channel with energy E4. This new signal energy E4 adds to the aggregate interference experienced by the first through third mobile stations already in the cell. Accordingly, in order to satisfy the required signal-to-interference ratios of the first through third stations, the power associated with the first three mobile stations E1–E3 may have to be adjusted accordingly. The same disruptive effect may be experienced when a mobile station which was previously located within the boundaries of the radio communication cell switches from a passive state to an active state to transmit or receive a message on the common channel.
The steady state condition is also disrupted when a mobile station leaves the radio communication cell. For example, if the steady state condition shown in FIG. 1A is disrupted by the third mobile station leaving the radio communication cell, the signal-to-interference ratio of the remaining two mobile stations are improved by the absence of the energy E3 on the common channel, as shown in FIG. 1C. Accordingly, the power of signals E1–E2 can be decreased to ensure efficient use of the common communication channel. Again, this same effect may be achieved when the third mobile station within the radio communication cell switches from active to passive state (e.g. by terminating its call).
Still another disruption of the steady state may occur when one or more mobile stations within a radio communication cell changes its operating characteristics. For example, as illustrated in FIG. 1D, if the third mobile station switches from a low data-rate mode of communication to a high data-rate mode of communication, the remaining two mobile stations within the cell experience increased levels of interference. To counteract the increased levels of interference in the communication band, the system may have to adjust the power levels E1 and E2. The reverse effect may occur when a mobile station switches from a high data-rate mode to a low data-rate mode.
Prior CDMA-based systems use one or more power control loops to appropriately adjust the power levels of signal transmissions within the system in order to counteract the above-described disruptions to the steady state condition. According to one exemplary prior technique, for the downlink the mobile station monitors the strength at which it receives signals from the base site. If the signals are too weak, the mobile station transmits a message to its associated base station informing the base station to increase the power at which it transmits to the mobile station. The base station responds accordingly. However, over time, the base “teases” the mobile station by slowly decreasing the power to the mobile station until the base station is informed by the mobile station to once again increase the power of transmission to the mobile station. This ensures that the base station is not communicating with the mobile stations using power levels which are unnecessarily high.
For example, in the case of FIG. 1B where a fourth mobile station enters a cell, the other mobile stations may instruct the base station to increase the level of power to the mobile stations. The base station responds accordingly by increasing the power by one increment. If still insufficient to satisfy the mobile station's SIR requirements, i.e., the signal-to-interference after the combiner stage in the base station, the mobile stations repeat their message to the base station, once again requesting the base station to increase the level at which it transmits messages to the mobile stations. This procedure may be repeated through a series of communications between the base and the mobile stations. If the base “overshoots” the power requirements of the mobile stations, it may have to decrease the power levels to the mobile stations.
In order to achieve and maintain this desired steady state condition, a method is needed to quickly and accurately measure a mobile station's signal-to-interference ratio. In a system employing wideband code division multiple access (WCDMA), channels are commonly transmitted from one base station using orthogonal channelization codes and the same scrambling codes; therefore, in making a SIR determination in such a WCDMA system, one should not only consider the interference from the base station which is orthogonal to the wanted signal, but also the interference from other base stations which is non-orthogonal to the wanted signal. When a signal is despread and combined at a mobile station, one skilled in the art will appreciate that the interference that is orthogonal to the wanted signal does not corrupt the signal. However, this cannot be said of the interference that is non-orthogonal to the wanted signal. Therefore, it is important to consider this non-orthogonal interference in estimating the mobile station's SIR.
Several methods exist for measuring the interference power associated with a received signal. According to a first approach, the searcher of the RAKE receiver generates an impulse response estimate of the received signal. The impulse response estimate is produced by, for example, match filtering the received signal with a signal that is known by the receiver to be included in the transmitted signal (i.e., the pilot signal). All received power, excluding the peaks, is used to estimate the interference. This concept is graphically depicted in FIG. 2, but suffers from the drawback of being slow and is also insensitive to the orthogonality or lack of orthogonality of the interference.
A second approach involves estimating the received power after despreading and combining of the pilot symbols. Assuming that the received signal power is constant for some period of time, the variance of the interference corrupting the signal can be estimated. This estimation is accomplished by using the mean value of the received despread and combined pilot symbols to calculate the distance from each sample of despread and combined pilot symbols to this mean value. Using these “noise vectors”, the variance of the interference can be estimated. This approach includes the orthogonality aspect if the measurement is done for all paths considered in the despreading process. However, it relies on the existence of a pilot symbol. If the pilot symbols are time-multiplexed, and thus discontinuously transmitted, the measurement can only be made when the pilot symbols are transmitted thereby resulting in intermittent SIR calculations. Another possible problem associated with this approach is that since the duty cycle is less than 100%, it may not be possible to do continuous estimation which may be required for, e.g., power control.
A third method for determining the interference associated with a received signal involves correlating the received signal with the channelization code allocated to the connection during a time when nothing is being transmitted to the mobile station. Since there is no “wanted” signal, despreading the received signal would then yield a good estimate of the interference. A problem with this approach is that the mobile station has to know when no information is being transmitted to it. This could be solved by having predetermined time-instants of no transmission, but such a solution has a certain capacity loss, since the interference measurement would need to be updated quite regularly.
In the above-identified, related application, a relatively fast automatic gain control (AGC) unit is implemented in a receiver that keeps incoming power to the RAKE at a constant level. This, in turn, enables the SIR to be calculated by multiplying the despread pilot symbol with a constant.
However, there continues to exist a problem in how to quickly and accurately measure interference in CDMA systems, particularly WCDMA systems, which takes into consideration signal interference which is both orthogonal and non-orthogonal to the wanted signal.